Spherical reflector

ABSTRACT

A dish-shaped acoustic reflector for use with a sound source, the reflector having a spherical surface for reflecting the desired echoes back toward the sound source in a narrow beam enhancing the strength, stability, and concentration of the received signal, said reflector accepting signals from sound sources over a predetermined angular range and wide frequency range.

United States Patent Palle G. Hansen; William E. Batzler, both of San Diego, Calif.

Dec. 16, 1968 Aug. 17, 1971 The United States of America as represented by the Secretary of the Navy Inventors Appl. No. Filed Patented Assignee SPHERICAL REFLECTOR 3 Claims, 4 Drawing Figs.

US. Cl 18l/0.5 A, 340/8 FT, 343/18 C Int. Cl G0lk 11/00 Field of Search. l81/0.5 A;

340/8 RT; 350/97, 296; 343/18 B, 18 C [56] References Cited UNITED STATES PATENTS 2,117,206 5/1938 Nef'f 350/97 X 3,243,768 3/1966 Roshon 340/10 3,458,853 7/1969 Daniels et a1. 340/3 Primary ExaminerRodney D. Bennett, Jr. Assistant Examiner-Daniel C, Kaufman Attameys--J. C. Warfield, Jr. and George J. Rubens ABSTRACT: A dish-shaped acoustic reflector for use with a sound source, the reflector having a spherical surface for reflecting the desired echoes back toward the sound source in a narrow beam enhancing the strength, stability, and concentration of the received signal, said reflector accepting signals from sound sources over a predetermined angular range and wide frequency range.

PATEN-TED-AUGl 719?! INVENTORS PALLE G. HANSEN By WILLIAM E. BATZLER FIG. 3

ATTORNEYS SPHERICAL REFLECTOR STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates to a compressional wave-reflecting device, and more particularly to a reflector of underwater acoustic signals.

Underwater sound reflectors have been employed for a variety of purposes, namely, as sonar targets for subaquatic aids to navigation, position markers for objects to be tracked by sonar, as a reference target for acoustic measurement, as well as other purposes. These reflectors have been constructed of various configurations. For example, there is the type referred to as a corner reflector, such as a triplane reflector fabricated of three intersecting planes. Other types of reflectors used in a two-way underwater transmission have been air-filled spheres. The prior developed reflectors all had a limitation in that they did not provide a high, stable target strength. For example, the air-filled sphere presented a relatively small effective reflecting surface such that small surface imperfections of this area cause large changes in intensity of the echo depending on the location of the active areaon the sphere. These reflectors also had the characteristic of being omnidirectional, that is, capable of reflecting signals from all directions. In addition, the air-filled sphere required heavy weighting means for anchoring the reflector to the ocean floor.

SUMMARY OF THE INVENTION The acoustic reflector of this invention has a concave spherical reflecting surface. The reflector surface may be uninterrupted, or, for some applications may be truncated to provide a central opening extending through the reflector. The spherical shape of the reflector satisfies the condition that signals incident on a certain annular zone of the hemisphere depending on the direction of the sound wave will be reflected back toward the sound source in phase and in parallel relation. Other signals not incident on this annular zone of the hemisphere, but still being reflected back toward the source, may be eliminated, if the particular application dictates, by providing the reflector with a circular opening, and these so-called other signals will be reflected through the opening and dissipated into the surrounding fluid medium, such as the sea water. Still other sound paths are reflected in directions other than those previously discussed and since they do not reach the receiver are of no further interest.

As a result of this novel construction, the reflector provides a highly stable target that produces a concentrated, high-level reflected signal, with a predictable good signature on a sonar receiver. This makes the reflector effective as a reference target or for other uses previously described.

STATEMENT OF OBJECTS OF THE INVENTION An important object of this invention is to provide a more effective reflector.

Another object is to provide a highly stable acoustic reflector having a high-level and predictable signature on a sonar receiver.

A further object is to provide an acoustic reflector that is directional, i.e., capable of returning signals received from within a predetermined angular range of directions, and a corollary object is to provide an echo having a narrow beam;

Still another object is to provide an acoustic reflector that is simple and inexpensive in construction and easily anchored to the ocean floor.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS I FIG. 1 is a perspective view of a typical underwater acoustic reflector constructed according to the teaching of the invention;

FIG. 2 is an enlarged vertical section of one reflector embodiment constructed of a solid reflector surface showing in solid lines the paths of two different reflected signal echoes;

FIG. 37 is an enlarged vertical section of another embodiment of the reflector provided with an opening formed by truncation, and showing in solid lines a desired signal echo being returned and in broken lines an unwanted signal being directed away from the receiver; and

FIG. 4 is a view of the reflector of FIG. 3 illustrating the directional aspects of the reflector and how the diameter of the opening formed by truncation determines the angle of acceptance between extreme positions of the sound source.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings where like reference numerals refer to the same part throughout the figures, FIG. 1 illustrates an acoustic reflector 10 comprising a dish-shaped reflector shell 12 fixedly or adjustably supported at 13 to a standard 14 for anchoring to a surface 16. It is apparent that the reflector may be supported by any conventional means depending on the particular application and environment. In the preferred embodiment, reflector 10 is being utilized as an underwater sonar target supported on the ocean floor, for two-way transmission between a conventional sound source and a conventional receiver, which for all practical purposes are the same transducer, neither the sound source or the receiver being illustrated as they form no part of this invention.

Reflector shell 12 is fabricated of any suitable rigid material, such as aluminum or the like which may be painted or otherwise coated for protection from the corrosive environment. As a practical matter for economy and simplicity, the reflector can be stamped, spun or otherwise made of sheet material.

The reflector is formed with a spherical surface 18 which terminates at one end in an opening I9 facing or directed toward the sound source so as to be capable of receiving a transmitted signal. The hemispherical surface 18 is an important feature of the invention in that it satisfies the conditions that certain incident signals from the sound source will be reflected in parallel relation and have the same path length, and therefore be in phase, so long as the signals are incident on a certain annular zone on the spherical surface.

Referring to FIG. 2, reflector 12 is of the type having a continuous spherical surface which faces or is directed toward a sound source that produces intermittent substantially plane sound waves through the water. Of the multitude of acoustic sound signals in the waves that are capable of being received by the reflector only two signals are illustrated, namely, A and B. Signals A and B are illustrated by spaced solid lines which are intended to represent a beam or bundle of sound waves but for purposes of description, each will be referred to as a signal. As signals A and B contain both transmitted and reflected parallel signals, arrows are provided on each respective signal in opposite directions.

Sound signal A is incident at 45on the reflecting surface at areas A1 and A2, the angle of incidence plus the angle of reflection of 45being equal to a total angle of as illustrated. The areas Al and A2 are part of a narrow spherical annular band or zone 20 on the reflector surface coinciding with an imaginary right circular cone X, Y, Z, having a vertex angle 21 of 90, the cone being tangent to the spherical surface at areas Al and A2. The longitudinal axis of cone X, Y, Z passes through the center 23 of the spherical surface and is perpendicular to the sound wave front creating signals A and B. For purposes of illustration, the width of zone 20 can be approximated by the width of the double lines of signal A. For the relationship illustrated in FIG. 2, signal A being reflected by zone 20 produces a uniquely concentrated echo that is of high signal strength when recorded by the receiver and easily identified from the other random signals received.

As previously described the transmitted sound wave contains many other signals incident on the reflector and include signals that are reflected back to the receiver and signals that are reflected away from the receiver, and for simplicity of illustration these signals that are being reflected back to the receiver other than signal A are collectively represented by signal B. Since signal B is incident on the spherical surface at three areas, namely, B1, B2, and B3, the angle of incidence plus the angle of reflection is greater than 90, as illustrated. The length of path of signal B is obviously longer than that of signal A, and, accordingly, signal B will reach the receiver later than signal A. For some uses of the reflector, the fact that both signals A and B are received in sequence and may not degrade the oscillograph recording because signal A being of highest intensity is the most predominant and is easily identifiable.

However, for some testing experiments, it has been found desirable to be'able to receive only signal A, and all other signals, such as B, being of a longer path may have a tendency to clutter reception of signal A. For such a reflector application, signal A may be considered the desired echo, and all other signals that are out of phase, such as signal B, may be considered unwanted echoes.

The present invention achieves this selectivity of received echoes in FIG. 3 by truncating a concave reflector 22 thereby providing a circular opening 24, being smaller and at an oppositely disposed end from opening 19. Geometrically, the resultant reflector structure can be defined as a zone of two bases. Now, in FIG. 3, the desired signal A is also illustrated as being reflected on the truncated reflector 22 in the identical manner as did reflector in FIG. 2 since the signal is reflected by the same zone between areas A1 and A2. As in FIG. 2, signal A is illustrated by solid spaced lines and the arrows are oppositely directed indicating the bundle of sound rays contains both transmitted and reflected signals. However, unwanted signal B can be reflected only at B1 and B2 and through the opening 24 and dissipated in the sea water. It is apparent that area B3 on the reflector 10 of FIG. 2 has been eliminated by the formation of opening 24. Accordingly, since signal B cannot be reflected back toward the sound source it represents only transmitted signals and is illustrated in FIG. 3 by broken spaced lines with both arrows in the same direction coming from the sound source.

Thus, the modification in FIG. 3 that provides opening 24 in the reflector produces a refinement of the invention in that reflector 22 returns to the receiver desired signals, such as A, and eliminates all unwanted signals, such as B. A signal echo is thus received which is concentrated and of a high intensity uncluttered by any other signals.

The dimensions of the diameters of openings 19 and 24 are not critical, but within limits the directivity of the reflector can be tailored to specific applications by selectively varying these diameters. The diameter of opening 24 cannot be larger than the base ofthe right circular cone x, Y, Z (FIG. 2) where it intersects areas Al and A2 since at least a portion ofthe annular spherical zone 20 would be eliminated to prevent the maximum return ofsignal A to the receiver.

There is no minimum critical diameter of opening 24, and, in fact, as illustrated by reflector 10 of FIG. 2 there need not be any opening in the reflector. However, if the reflector is to be used as a directional subaquatic aid for navigational purposes the size of opening 24 will determine the directional characteristic of the reflector. Maximum target strength is obtained within an angle of acceptance 26 as shown in FIG. 4.

The configuration of the reflector need not be any greater than a hemisphere as it may otherwise complicate fabrication of the reflector without any benefit. The dimensions of both openings 19 and 24 are best described with reference to FIG. 4.

FIG. 4 illustrates reflector 22 of the embodiment described in FIG. 3 which will return an echo of maximum intensity back to the sound source in any position or zone between a desired sound signal represented by C and a desired sound signal represented by D. The annular zones of reflection on the reflector, corresponding to zone 20 in FIGS. 2 and 3, are represented by areas Cl and C2, and D1 and D2, respectively, the angle between these beams being the angle of acceptance 26.

It is apparent that with the given reflector structure illustrated in FIG. 4 all signals originating from the sonar outside this angle of acceptance would not be reflected back to the receiver at equally high levels because a portion of the annular zone represented at either area C1 or D1 would be off the reflector and part of the signal would pass through opening 24 and be dissipated in the sea water which would cause a reduction in signal intensity below the maximum level. The diameter of opening 19 need not be any greater than the diameter between D2 and C2 for the remaining reflecting surface of the reflector is not utilized.

Should it be required that the angle of acceptance of the reflector be larger, then the reflector can be constructed with a correspondingly smaller diameter opening 24 to the extent that areas C2 and D2 remain on the reflector, otherwise a portion of the signal will be reflected into the sea water environment and be dissipated.

As previously described, as the diameter of opening 24 is constructed to approach the diameter of annular zone C1, C2 or D1, D2 the angle of acceptance is correspondingly reduced and the zone in which the reflector will return an echo of maximum intensity is correspondingly decreased. In this manner the reflector can be constructed to reflect a high-level signal within a desired zone depending on any given requirements of the reflector installation.

The hemispherical reflector according to the present invention produces a reflected signal which is concentrated and has a high-level and predictable signature on a sonar receiver. Tests have indicated that over a frequency range of at least 50 to 200 kHz. maximum echo signal received by the invention reflector has a 20 db. higher signal strength than the prior art air-filled sphere, or in other words the echo amplitude by the reflector of the instant invention is 10 times as great. In addition, the reflected signal will come back in a very narrow beam (at kHz. at a beam width of about 2) while capable of being returned over a wide angle of acceptance. The practical advantage of such a narrow beam is that when the reflector is used as a subaquatic aid that the receiver of one vessel will not receive echoes from transmitted signals from other vessels. This advantage cannot be achieved by the commonly used prior art reflector such as the air-filled sphere which echo is omnidirectional. The reflector can be employed in different applications, and if it is desired that the returned signal be uncluttered with echoes having different path lengths the reflector can be truncated to provide a means for dissipating undesired echoes. The novel reflector is simple and inexpensive in construction. For subaquatic aids to navigation the zone of acceptance of each reflector can be tailored to meet the desired requirements. Where the reflector is truncated an additional collateral advantage is the reduction of lateral resistance to ocean currents reducing the strength required of the supporting structure.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the concept of the present invention, the invention may be practiced otherwise than as specifically described.

What we claim is:

1. A reflector for receiving and returning signals transmitted by a transducer located in a given transmitting medium, the transducer being remotely positioned and distinct from the reflector;

said reflector is provided with a circular aperture extending therethrough;

said aperture being said means for dissipating said signals each having an angle of incidence and an angle of reflection the sum of the angles being greater than a right angle.

3. The reflector of claim 2 wherein:

the diameter of said circular aperture is no greater than the base of .a right angle cone intersecting the point of tangency of the cone with the reflector. 

1. A reflector for receiving and returning signals transmitted by a transducer located in a given transmitting medium, the transducer being remotely positioned and distinct from the reflector; said reflector having a concave spherical surface no greater than a hemisphere in contact with said transmitting medium, said surface adapted to return a signal from said transducer; means for dissipating signals received by the reflector having an angle of incidence and an angle of reflection the sum of which angles are greater than a right angle; whereby certain signals received by the reflector will be incident on the spherical surface and be returned to the transducer along parallel paths.
 2. The reflector of claim 1 wherein: said reflector is provided with a circular aperture extending therethrough; said aperture being said means for dissipating said signals each having an angle of incidence and an angle of reflection the sum of the angles being greater than a right angle.
 3. The reflector of claim 2 wherein: the diameter of said circular aperture is no greater than the base of a right angle cone intersecting the point of tangency of the cone with the reflector. 